Providing that the board shall recommend to the Secretary and the Secretary shall fix the assessment rate at not more than 2 cents per one hundred pounds of potatoes handled; except that if approved by producers, and importers when importers are subject to a plan, pursuant to section
2623 of this title, the rate of assessment shall not exceed one-half of 1 per centum of the immediate past ten-calendar-year United States average price received for potatoes by growers as reported by the Department of Agriculture.